STRUCTURAL AND MICRO-STRUCTURAL ANALYSIS OF SHEAR ZONES IN THE ZAGROS OROGENY, Dehbid area, southwestern, Iran

Crustal-scale shear zones usually accommodate most of the imposed deformation in orogenic collision zones. Thus the kinematic reconstruction within such shear zones is one of the most effective ways to constrain key aspects to tectonic evolution of orogenic systems. Deformation in a shear zone causes development of characteristic fabrics and mineral assemblages that reflect P-T conditions, flow type, movement sense and deformation history in the shear zone. As such, shear zones are an important source of geological information. The Zagros Thrust System constitutes an excellent example of a crustal shear zone. Structural, micro-structural and kinematic studies of the HP–LT Sanandaj-Sirjan metamorphic belt in the Dehbid area indicate a progressive evolution. All these changes are compatible with progressive increase of ductile shear strain and dynamic recrystallization during transpressional deformation. Quantitative finite strain (Rxz = 2-4.7) and kinematic vorticity number into W (mean W-value, 0.76±0.14) analyses demonstrate that both the strain ratio and the simple shear component of deformation increase towards the Main Zagros Thrust (i.e. from northeast to southwest). The variation in quartz c-axis patterns is explained in terms of decreasing metamorphic grade during ductile deformation from the SW (high to medium grade) to the NE (low grade), and decreasing finite strain respectively which is in accordance with the general progression of transpressional tectonics. The studied quartz c-axis fabrics display opening angles ranging from 54° to 66°.Using this geo-thermometer, these angles indicate deformation temperatures of 460  50°C, 545  50°C, The c-axis patterns thus indicate that the quartzites were deformed under greenschist to epidote-amphibolite facies conditions. Spatial orientation of lineation and semi-quantitative kinematic indicators indicate shear zone parallel stretching and shear zone perpendicular shortening. Structural characters of the investigated deformation zone indicate that it is a dextral oblique transpresional zone with triclinic symmetry. Presences of dextral-shear sense indicators indicate that the Zagros Thrust zone is a dextral inclined transpressional zone and shear zones in this system are the sole thrusts.